This invention relates in general to data processing systems and in particular to a video data recovery and expansion system for use in connection with a digital high definition television system. The recently tested high definition television system of Zenith Electronics Corporation and AT&T Corporation includes a video compression scheme for compressing 37 MHz information for transmission over a 6 MHz wide television channel.
U.S. Pat. No. 5,285,276 describes a temporally oriented video compression system in which compressed video information is transmitted in the form of motion vectors and difference signals with the motion vectors identifying previous portions of a frame of video that closely match the current portion and the difference signals representing the differences between the previous and current portions. The video information has a bandwidth of about 37 MHz and may comprise a progressively scanned video signal in the form of successive frames of binary video data having a vertical periodicity equal to the NTSC standard (i.e. about 59.4 Hz) and a horizontal periodicity equal to three times the NTSC standard (i.e. about 47.25 KHz). The data is in the form of a series stream of binary pixel values that have been transform coded in the frequency domain to develop discrete cosine transform coefficients. The transform coding process provides a series of clusters of spectral transform coefficients for each frame of video, with each cluster of coefficients corresponding to a different spatial region of the video image. Each cluster, for example, may comprise an 8.times.8 array of coefficients with 14,400 clusters representing an entire video frame. The coefficient clusters are serially applied to a perceptual modelling system which develops an output that reflects the perceptual nature of the corresponding portion of the video image. The video data is then compressed in accordance with a selected compression algorithm. One well-known compression technique does not send all of the transform coefficients, the coefficients whose omission will have the least noticeable effect on the received image being dropped. The remaining coefficients are variable length encoded and sent as a series of codewords of unequal bit length, with the shortest codewords being assigned to those values that are most probable.
As further discussed in U.S. Pat. No. 5,285,276, the data may be ranked by importance, i.e. control data may be sent in more robust initial data segments, followed by data of the next level of importance such as motion vectors, etc. It will be noted that the number of motion vectors and difference signals may vary from frame to frame depending upon the perceptual nature of the video information in the frame and its relationship to the previous frame. The compressed variable length encoded information is assembled into Blocks consisting of a fixed number of 8.times.8 coefficient arrays, from most of which some coefficients have been omitted. Included with each Block is a Block header referred to as a selector, that consists of a fixed number of variable length encoded codewords containing the selector information. The selector information identifies which coefficients have been omitted from the 8.times.8 arrays in the Block and the total number of coefficients in the Block. The Blocks are assembled into data frames, each frame comprising a preselected number of equal length data segments. Each data segment has an initial fixed length segment sync portion, a fixed length data segment header and Block data, i.e. selector and coefficient data. The data segment header has a pointer that indicates the location in the data segment where the first Block beginning, if any, in that data segment occurs. Thus the selector data and coefficient data may be recovered by counting codewords and coefficients.
It will be noted that since the size of a Block is variable and the size of a data segment is fixed, the number of Blocks in a data segment will vary in accordance with the amount of compression. Therefore, several Blocks may be contained in a single data segment, or a single Block of data may extend over several data segments. As to the variable length encoding of the Block data, any of a well known number of encoding systems may be used such that the data or codewords may be joined, i.e. sequentially transmitted without breaks therebetween. The receiving system can produce a state table for determining the boundaries or junctions between adjacent codewords. The encoding form known as Huffman encoding is presently preferred. This is all by way of background to the present invention which will be understood not to be restricted to any particular form of encoding or processing.
The problem solved by the present invention is caused by the high data rate of the expanded data, which requires the buffer memories to have very high read speeds. Such high read speeds are attainable only at very high cost. The present invention is directed to a system for significantly reducing the need for buffer memories requiring high reading speeds.